The present invention relates to a procedure to form a non-woven cloth made of synthetic filaments in which synthetic filaments are extruded, stretched, cooled, and projected onto a plastic fabric that is in movement. Known procedures are the SPUNBONDED and MELTBLOWN procedures.
To apply these already known procedures, extrusion dies are used which are placed above a forming fabric. Thermoplastic polymers, such as polyester, polyamide, polyolefins, etc. are hot-extruded. By means of a violent air current, the filaments are stretched, cooled and projected onto the endless forming fabric, which moves at a constant speed, to form a non-woven cloth. A suction system under the forming fabric makes it possible to eliminate the air pockets and to adhere the cloth against the forming fabric.
When the system is turned on or off, such as may result from malfunctions, it can happen that drops of melting polymer from the extruder fall on the fabric and become embedded in the forming fabric. This damages the quality of the cloth and sometimes causes the removal of the forming fabric because the cloth filaments may cling to these impurities, and the cloth will be poorly formed if the clogged areas are too big.
The use of plastic forming fabrics is already known. Because of their flexibility, they make installation easy and are easily guided, but it is not possible to clean them, and consequently, their life span is relatively short. If we use heat to soften of felt impurities fabrics will shrink or melt locally which is not acceptable. For instance this is the case when running polyester monofilament fabrics to make polypropylene non-woven.
Thus, the use of metallic fabrics, despite their heaviness and stiffness, which makes installation difficult and time consuming and, when in operation, their tendency to accidentally crease, has received wide acceptance.
In fact, the only real advantage of metallic fabrics is that they can be cleaned relatively easy by using a hot, high temperature source, such as open flame. But heat tends to expand fabric locally creating creases which are not acceptable.
According to the present invention, a forming fabric made of special plastic filaments is used, the melting point temperature, t.sub.1, of these special filaments is at least 80.degree. C. higher than the melting point temperature, t.sub.2, of the filaments that constitute the non-woven cloth; this will make it possible to clean the forming fabric by passing a hot cleansing fluid, i.e. steam or hot air, over it at high speed, the temperature of the cleansing fluid is controlled and lies between t.sub.1 and t.sub.2 and should preferably be ##EQU1##
The invention also relates to forming fabrics that allow the application of the procedure.
Suitable forming fabrics can be made of a polyphenylene sulfide (t.sub.1 =280.degree. C.) which is a relatively cheap material. In such a case, polymer filaments extruded from a material with a melting point of t.sub.2 no greater than 200.degree. C. will be used to form the cloth. One such family of materials suitable for use in this case is polyolefins.
In a higher temperature application of the invention, one can use fabrics made of polyetheretherketone which has a melting point t.sub.1 around 330.degree.. In this case, polymer filaments materials such as polyester and polyamide with a melting point around 230.degree. C. can be used to form the cloth.